Structure of hardwoods
A block of hardwood, magnified 250 times
Botanically speaking, hardwoods are angiosperms. This literally means 'covered seeds', and refers to the fact that the seeds are contained in gumnuts or flowers.
Most hardwoods have two main types of cells that run vertically. Sap is carried upwards in the pores (also called vessels) which join together one on top of another to form pipes. Strength is provided by fibres, which have thick cell walls and make up the bulk of the wood.
There are also cells called rays, which extend outwards from the pith towards the bark. Ray cells act as food storage areas in the stem. Although they are generally too small to see without a lens, some hardwood species have very large rays, giving timbers such as silky oak quite a distinctive appearance.
Cell walls are made mostly of cellulose, and are cemented together with a glue-like substance called lignin.
The term 'hardwood' often confuses people who don't know much about the structure of wood. This is because people naturally think that all hardwoods must be 'hard'. The fact is, they're not all hard. Some species, like balsa, are actually very soft.
The bottom line for classifying a timber as either a softwood or hardwood is whether or not it contains pores. In other words:
- all timbers with pores are hardwoods
- all timbers without pores are softwoods.
Some people think that it would be much easier to abolish the terms 'hardwood' and 'softwood' and simply refer to different species as either 'pored' or 'non-pored'. But that's never likely to happen, because these common terms are used by everyone, and it is true that often they are quite accurate in describing the hardness of the timber in question.
The learning activity for this lesson is combined with the next lesson. It's a test to see how well you can classify various commonly-used species as either hardwood or softwood. So you'll need to go to the next lesson first, before you do the activity.